Compiling BOM-VECTOR emits a "type assertion to complex to check note." The reason is that (THE FOO (QUUX)) is actually of type (VALUES FOO &REST T) which SBCL, at least for the moment, cannot cope well with. The below patch changes it to (THE FOO (VALUES (QUUX))). -T. diff -rN -u old-babel/src/strings.lisp new-babel/src/strings.lisp --- old-babel/src/strings.lisp 2009-06-01 14:58:56.000000000 +0200 +++ new-babel/src/strings.lisp 2009-06-01 14:58:56.000000000 +0200 @@ -237,15 +237,16 @@ (defun bom-vector (encoding use-bom) (check-type use-bom (member :default t nil)) (the simple-vector - (if (null use-bom) - #() - (let ((enc (typecase encoding - (external-format (external-format-encoding encoding)) - (t (get-character-encoding encoding))))) - (if (or (eq use-bom t) - (and (eq use-bom :default) (enc-use-bom enc))) - (enc-bom-encoding enc) - #()))))) + (values + (if (null use-bom) + #() + (let ((enc (typecase encoding + (external-format (external-format-encoding encoding)) + (t (get-character-encoding encoding))))) + (if (or (eq use-bom t) + (and (eq use-bom :default) (enc-use-bom enc))) + (enc-bom-encoding enc) + #())))))) (defun string-to-octets (string &key (encoding *default-character-encoding*) (start 0) end (use-bom :default)